This invention relates to vehicles of the amphibious air cushion vehicle (ACV) type; and more particularly to improvements in propulsion propeller therefor. Such vehicles typically employ aerodynamic type propellers operating at maximum safe rotational speeds for driving such vehicles over water, land, swamp, or ice surfaces, or the like; and the present invention is specifically directed to combating the extraordinarily severe propeller blade shape erosion problems encountered by ACV type vehicles during typical operational missions, whereby they are called upon to travel over a variety of disparate terrain conditions.
For example, when such vehicles are being employed for use in precisely scheduled commercial transport programs or for transport of supplies or troops from off-shore carriers onto a land-based beach or the like, the blades of the vehicle propulsion propellers are alternately subjected to ingestions of sprays of salt water and rain water "droplets" and air streams carrying sand (or other terrain-based particular materials). The hyper-velocity impactions of such droplets and/or solid particulates against the rapidly rotating propeller blades result in unduly rapid erosion of the initially efficient blade section profiles at the tip and leading edge portions thereof, resulting in serious propeller performance deterioration problems.
The problems of impeller/propeller blade erosion when operating in various abrasive environments such as in the slower moving turbine engine and pump industries has long been encountered, and a variety of means for amoring the blades have been devised and employed with some success. Such means typically include leading edge overlays of stainless steel or coatings of abrasion resistant hard metals or elastomers, and the like. Such devices as well as ways and means for attaching them to the blades are disclosed for example in U.S. Pat. Nos. 1,050,488; 1,255,650; 1,335,002; 3,315,941 and 3,844,728. However, the present invention relates to the protection of propeller blades of the type which are cantilever rooted in a propeller hub and extend therefrom radially in elongated form, whereby they are subjected in normal operation to forces compelling them to undergo cyclical bending and axial twisting gyrations.
Reference to and proposed solutions to such problems to which the present invention relates appear in Technical Report AFML-TR-69-287, PART II, John W. Morris, Jr., Sept. 1969, and "TEXTRON'S BELL AEROSPACE CO. TECHNICAL REPORT" AFML-TR-265, John W. Morris, Jr. and Norman E. Wahl, Nov. 1970. These publications elaborate upon and are useful in understanding the problems confronting the research resulting in the present invention.
As explained hereinabove, propellers of ACV type vehicles designed for missions as above described are typically subject to not only a cost-effectiveness problem, but are in a unique category because in event propeller failures occur after only a few hours of operation and before completion of a timed delivery commitment, it may be necessary to abort the mission. Accordingly, the search for suitable means for ensuring a substantially extended efficient operational life of a propeller blade for employment in such situations has now become a top priority project. However, in order to advance this art, the abrasion guard system needs not only to be of maximum erosion resistance, but must also be of such characteristics as are uniquely qualified to be adapted to accommodate and service against the encountered blade twisting and bending gyrations, as well as being compatible with the physical and chemical properties of the material from which the blades per se are constructed.
Therefore, the object of this invention was to provide an improved propeller blade of the type which basically comprises a strong aluminum alloy, or a resinous matrix with reenforcing inclusions and which is armored in combination therewith by highly resistant to corrosion means of such physical and chemical properties as to avoid stress concentrations at the blade and armor interface; whereby the composite blade would be of markedly improved operationally efficient extended life capability.
It was earlier recognized that ceramic materials with glassy, smooth surfaces have a basic advantage over most other materials for the purpose of shunting away particles impinging thereagainst, but also that most inorganic ceramics tend to brittlize and fracture under fairly low impact loads. Accordingly, research was initially directed to the basic problem of finding an erosion resistant ceramic such as would overcome the tendency as in the case of most inorganics to fail brittlely under fairly low impact loads. However, it was also noted that inasmuch as aerodynamic propeller blades of the type to be dealt with are relatively long and slender and twist and bend when in operation (as distinguished from turbine type blades), for use in accordance with the present invention the erosion armor arrangement for such purposes must be able to accommodate differential stress fluctuations along extended impact surfaces without tensile failure. Also, it must be of adequate yield strength in order to ensure suitable elastic response to the compressive loads imposed thereon by impacts of the order encountered in this art. Subsequent research, trials and tests of a large variety of combinations of basic blade and implant guard materials resulted in discovery of the present invention; and as a result propeller blades are now being fabricated which successfully survive the operating efficiency requirements therefor some four times longer than blades embodying prior art armored blade proposals.